The present invention relates to a method of automatically focussing a video pickup device. The pickup device has an objective and produces a digital image. The invention also relates to a video camera in which this method can be used.
The invention is particularly advantageous for use in any video camera in which the operating conditions necessitate frequent focussing corrections, and more specifically for infrared cameras whose germanium optical system has a refractive index, and consequently a focal length which varies with temperature.
An automatic focussing device for video cameras is disclosed in U.S. Pat. No. 4,392,726. In this prior art device, the photo-detecting surface of the camera is positioned such that a d.c. detection signal, integrating the high-frequency components of the image, is maintained at a maximum value. After sampling, the detection signal is differentiated in an analog manner. Thereafter the resultant differential signal is compared with a threshold.
This operation attempts to define a point near the maximum of the detection signal by the point at which the sampled difference signal starts to exceed the threshold. However, taken the sampling operation and the comparison at the threshold into account, this point thus determined does not accurately coincide with the zero point of the difference signal and is consequently not representative of the maximum of the detection signal.
The prior art device further includes means which move the photosensitive surface of the camera from the previously determined position to the position giving the maximum value of the detection signal.
The device of U.S. Pat. No. 4,392,726 has a certain number of disadvantages. It requires a comparatively complex, essentially analog processing chain for the incoming video signal, comprising filtering, integration, sampling, differentiation, comparison, etc. All of these operations, however, are not adequate to produce perfectly satisfactory automatic focussing control, since an operation for correcting the systematic error produced by the device itself has to be added to the above operations. Further, this prior art device is not effective unless the image contains sufficient high-frequency components, which is not the case then the filmed scene has only few details.